Although cellular receptors for immunoglobulins were first identified nearly 40 years ago, their central role in the immune response was only discovered in the last decade. They are key players in both the afferent and efferent phase of an immune response, setting thresholds for B cell activation and antibody production, regulating the maturation of dendritic cells and coupling the exquisite specificity of the antibody response to effector pathways, such as phagocytosis, antibody dependent cellular cytotoxicity and the recruitment and activation of inflammatory cells. Their central role in linking the humoral immune system to innate effector cells has made them attractive immunotherapeutic targets for either enhancing or restricting the activity of antibodies in vivo.
The interaction of antibodies and antibody-antigen complexes with cells of the immune system effects a variety of responses, including antibody dependent cell-mediated cytotoxicity (ADCC) and complement dependent cytotoxicity (CDC), phagocytosis, inflammatory mediator release, clearance of antigen, and antibody half-life (reviewed in Daron, Annu Rev Immunol, 15, 203-234 (1997); Ward and Ghetie, Therapeutic Immunol, 2, 77-94 (1995); Ravetch and Kinet, Annu Rev Immunol, 9, 457-492 (1991)), each of which is incorporated herein by reference).
Antibody constant domains are not involved directly in binding an antibody to an antigen, but exhibit various effector functions. Depending on the amino acid sequence of the constant region of their heavy chains, antibodies or immunoglobulins can be assigned to different classes. There are five major classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM, and several of these may be further divided into subclasses (isotypes), e.g., IgG1, IgG2, IgG3, and IgG4; IgA1 and IgA2. The heavy chain constant regions that correspond to the different classes of immunoglobulins are called α, δ, ε, γ, and μ, respectively. Of the various human immunoglobulin classes, human IgG1 and IgG3 mediate ADCC more effectively than IgG2 and IgG4.
Papain digestion of antibodies produces two identical antigen binding fragments, called Fab fragments, each with a single antigen binding site, and a residual “Fc” fragment, whose name reflects its ability to crystallize readily. The Fc region is central to the effector functions of antibodies. The crystal structure of the human IgG Fc region has been determined (Deisenhofer, Biochemistry, 20, 2361-2370 (1981), which is incorporated herein by reference). In human IgG molecules, the Fc region is generated by papain cleavage N-terminal to Cys, 226.
IgG has long been appreciated to mediate both pro- and anti-inflammatory activities through interactions mediated by its Fc fragment. Thus, while Fc-FcyR interactions are responsible for the pro-inflammatory properties of immune complexes and cytotoxic antibodies, intravenous gamma globulin (IVIG) and its Fc fragments are anti-inflammatory and are widely used to suppress inflammatory diseases. The precise mechanism of such paradoxical properties is unclear but it has been proposed that glycosylation of IgG is crucial for regulation of cytotoxicity and inflammatory potential of IgG.
IgG contains a single, N-linked glycan at Asn297 in the CH2 domain on each of its two heavy chains. The covalently-linked, complex carbohydrate is composed of a core, biantennary penta-polysaccharide containing N-acetylglucosamine (GIcNAc) and mannose (man). Further modification of the core carbohydrate structure is observed in serum antibodies with the presence of fucose, branching GIcNAc, galactose (gal) and terminal sialic acid (sa) moieties variably found. Over 40 different glycoforms have thus been detected to be covalently attached to this single glycosylation site. Fujii et al., J. Biol. Chem. 265, 6009 (1990). Glycosylation of IgG has been shown to be essential for binding to all FcyRs by maintaining an open conformation of the two heavy chains. Jefferis and Lund, Immune.l Lett. 82, 57 (2002), Sondermann et al., J. Mol. Biol. 309, 737 (2001). This absolute requirement of IgG glycosylation for FcyR binding accounts for the inability of deglycosylated IgG antibodies to mediate' in vivo triggered inflammatory responses, such as ADCC, phagocytosis and the release of inflammatory mediators. Nimmerjahn and Ravetch, Immunity 24, 19 (2006). Further observations that individual glycoforms of IgG may contribute to modulating inflammatory responses have been suggested by the altered affinities for individual FcyRs reported for IgG antibodies containing or lacking fucose and their consequential affects on cytotoxicity. Shields et al., J. Biol. Chem. 277, 26733 (2002), Nimmerjahn and Ravetch, Science 310, 1510 (2005). A link between autoimmune states and specific glycosylation patterns of IgG antibodies has been observed in patients with rheumatoid arthritis and several autoimmune vasculities in which decreased galactosylation and sialylation of IgG antibodies have been reported. Parekh et al., Nature 316, 452 (1985), Rademacher et al., Proc. Natl. Acad. Sci. USA 91, 6123 (1994), Matsumoto et al., 128, 621 (2000), Holland et al., Biochim. Biophys. Acta Dec 27; [Epub ahead of print] 2005. Variations in IgG glycoforms have also been reported to be associated with aging and upon immunization, although the in vivo significance of these alterations have not been determined. Shikata et al., Glycoconj. J. 15, 683 (1998), Lastra, et al., Autoimmunity 28, 25 (1998).
Accordingly, there is a need for the development of methods for the generation of polypeptides that would account for the disparate observations of IgG properties in vivo.